Triggered spark gap



Oct. 12, 1965 R. E. HUESCHEN 3,211,940

TRIGGERED SPARK GAP Filed Dec. 29. 1960 ROBERT E.HUESCHEN INVENTOR.

United States Patent 3,211,940 TRIGGERED SPARK GAP Robert E. Hueschen,Hales Corners, Wis., assignor to General Electric Company, a corporationof New York Filed Dec. 29, 1960, Ser. No. 79,445 7 Claims. (Cl. 313205)This invention relates to are discharge devices and more particularly,to triggered spark gap tubes.

The triggered spark gap is an arc discharge device that functions as ahigh voltage switch; it employs the electrical breakdown of a gas withinthe tube in order to provide conduction. Triggered spark gaps have beenused for switching up to at least 30,000 volts. They are especiallyuseful in electrical circuits that require extremely fast and veryreliable switching functions. Typical applications are those whereinswitching times must be less, and often considerably less, than onemicrosecond. One such application of the triggered spark gap isdisclosed in the United States Patent No. 2,867,728 to H. C. Pollock forLogging Apparatus which issued on January 6, 1959. Shown therein is acircuit wherein the spark gap is utilized for pulsing a source ofneutrons used for lithological logging analyses. The specificrequirement of the logging apparatus of that patent is for -a highvoltage pulse which is a small fraction of a microsecond. The triggeredspark gap is well suited for this type of switching function.

In essence, the triggered spark gap is a tube, the interior of which isfilled with a gas which is capable of ionization when an appropriatevoltage is applied between certain of the electrodes. The triggeredspark gap tube comprises three electrodes. Two of them act as thecontacts of a switch, and the third acts to close this elemental switchby virtue of breaking down or ionizing, with a preparatory or initiatingpulse, the gas disposed between the third electrode .and one of theother two electrodes. The two electrodes which act as the contacts ofthe switch are called dome electrodes because they are hollowhemispherical shapes. The dome electrodes are disposed opposite oneanother, and are spaced apart (and insulated from each other) by adistance which is appropriate for the voltage of interest. The thirdelectrode (which is shaped like a tube or rod and is called the triggerelectrode) projects just through, and is insulated from, the surface ofone of the two dome electrodes. When the spark gap is to be switched, ashort pulse is applied between the trigger and one of the two domeelectrodes. This diiference of potential is sufiicient to break down thegas between these two closely spaced electrodes, whereby ionizationtakes place therebetween. One of the theories advanced to explain thetriggering of the triggered spark gap is that the ultraviolet radiationwhich is generated by this localized breakdown serves to trigger theionization of the rest of the gas between the two dome electrodes. Thisresults in an avalanche breakdown or ionization of the gas between thedome electrodes. The ionized gas serves as a conductor thereby toelfectively close the switch. The term breakdown as applied to sparkgaps means electrical conduction of the gas between the electrodes. Inthis way, the triggered spark gap acts as a high voltage switch.

Three of the most important conditions for successful operation of aspark gap should be understood for this discussion. The first conditionis that breakdown (i.e., conduction) must not occur as long as thepotential applied between the dome electrodes is below a certain value.This value is frequently called the hold-off value. Spark gaps may beconstructed having hold-off values ranging from 500 to at least 30,000volts. The

3,211,940 Patented Oct. 12, 1965 trigger electrode is not energizedwhile the hold-oft value is being determined or while the spark gap isbeing subjected to a specified hold-off test.

Secondly, breakdown between the dome electrodes must occur when thepotential across them is greater than a certain minimum value. In otherwords, if the difference of potential between the two dlorne electrodesis great enough, the gas therebetween should break down, and conductionbetween the electrodes should follow. The value at which the breakdownshould occur (keeping in mind that the trigger electrode is notenergized at this time either) is often termed the minimum static breakdown voltage. The static breakdown voltage of spark gaps is usually notless than 25% greater than the holdoif value.

Thirdly, the breakdown must occur when the potential applied across thetwo dome electrodes is within a certain specified range between thehold-01f and static breakdown values when the trigger electrode isenergized. Furthermore, this breakdown must occur within a certain, andvery short, period of time after the trigger electrode is energized.This delay period between application of the pulse to the triggerelectrode and the breakdown between the two dome electrodes is calledthe delay time. This is usually a fraction of a microsecond. The averagetime variation that a spark gap tube may exhibit in its delay timeduring a series of pulses is called jitter.

Not only is the triggered spark gap a high voltage switch, but thecurrent that is switched can be consider able. Since it is an arcdischarge device, its conduction current is limited only by the constantof the external circuit. Such operation results in very hightemperatures at the cathode dome electrode because of the high currentdensity, and also in intense ionic bombardment of the cathode because ofthe high voltages. These two effects result in an erosion of the cathodedome electrode due to the vaporization of the cathode metal as a resultof the high temperatures and also due to the ionic bombardment. There isa subsequent and inevitable deposition of the vaporized electrode metalelsewhere (and undesirably) on the interior surfaces of the spark gaptube. The combined result of high temperature and ionic bombardment onthe cathode dome electrode is, for my purposes here, termed sputtering.

A metallic coating on the insulating materials that separate the threeelectrodes within the spark gap affects the electrical characteristicsof the spark gap in an undesirable way. Typically, a thin conductivecoating forms on the insulator that separates the two dome electrodes,and also on the insulator that separates the trigger electrode with itscoupled dome electrode. The dome electrode triggered with the triggerelectrode acts as the cathode in normal operation and is usually calledthe cathode electrode or trigger dome electrode. The other of the twodome electrodes is termed the main dome electrode. 0

Clearly, the more often the tube is fired or pulsed, the more sputteringresults, and as time goes on, the greater the erosion of the triggerdome electrode and deposition of the eroded metallic material on theinsulation of the spark gap. In this way, sputtering not only causesunacceptable performance of the tube with respect to the hold-off valueand the delay time, but the life of the tube is greatly reduced.

Indeed, sputtering has been so great a problem that prior to theutilization of the principles of the instant invention in connectionwith spark gaps, more than twenty-five per-cent of the tubes producedhad to be rejected as unacceptable for failing the hold-off, staticbreakdown, or delay and jitter tests, or because the life of the tubewas too short. With the applications of the.

principles in accordance with the instant invention, however, rejectionof manufactured tubes has been reduced to ab out ten percent.

It is the primary object of this invention, therefore, to provide animproved arc discharge tube characterized by longer life and betterperformance characteristics than has heretofore been possible, and whichfor any specified performance characteristic, may be manufactured tosatisfy those characteristics with a higher production yield than hasbeen heretofore possible.

The above object has been satisfied in accordance with the principles ofthe invention in a two-fold manner. Firstly, in accordance with theprinciples of the invention, it was discovered that sputtering of theelectrodes and deposition of the eroded metal onto the insulatingmaterial could be considerably reduced such that hold-off problemsbecame insignificant. This is accomplished by contouring the inside faceof the insulator separating the two dome electrodes to conform in shapeto the electric field pattern between the two dome electrodes (thatwould exist in free space) when voltage is applied thereto.

Secondlyyit has been found that by cutting a groove or gap in theinsulator separating the trigger electrode from the trigger domeelectrode, the possibility of a conductive short therebetween issubstantially eliminated, even when there is considerable sputtering anddeposition of eroded metal on the insulator between the trigger andtrigger dome.

The novel features believed to be characteristic of the invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings.

In the drawings:

FIGURE 1 is a perspective view with portions cut away of a triggeredspark gap tube in accordance with the invention;

FIGURE 2 is a longitudinal cross section of the triggered spark gap ofFIGURE 1 taken through a plane including the longitudinal-axis of thetube, so as to highlight the shape of the contoured inner surface of theinsulator between the two dome electrodes;

FIGURE 3 is a cross section of the triggered spark gap of FIGURES 1 and2 taken along lines 33, so as to highlight the grooved insulator betweenthe trigger electrode and the trigger dome electrode; and

FIGURE 4 is a portion of the tube showing the trigger, the trigger dome,and the grooved insulator therebetween after the tube has been pulsedmany times.

Referring with particular attention at this point to FIGURES 1 and 2,there is shown a triggered spark gap tube in accordance with theprinciples of the invention. So that a frame of reference may be had,the external diameter of the cylindrical body insulator 11 of the tubeof FIGURE 1 would be characteristically approximately .675 of an inchfor a typical tube. The cylindrical insulating body of tube 11 ispreferably a ceramic material to which the metal electrodes may bebrazed. A particularly satisfactory type of ceramic comprises ninetyfivepercent A1 with the remaining five percent comprising Cr O SiO MgO andCaO. A commercially available ceramic having this composition goes underthe trade name of Diamonite P3142-1, although I am not restricting theceramic either to this type or to this chemicalcomposition. It need onlysatisfy the requirement that it be a good insulator, strong, and easilyadaptable for brazing to metals such as tantalum or molybdenum.

The major functions of the ceramic body insulator 11 are: the formationof the envelope of the tube so that the gas may be contained therein ata desired pressure; and the electrical separation and insulation of thetwo dome electrodes 12 and 13. At the lower portion of the ceramic bodyinsulator 11. i a h l ow hemispherical metallic electrode 12. This isthe main dome electrode and in its usual operation normally has appliedto it a positive potential relative to the dome electrode 13. Located atthe top portion of insulator 11 is the trigger dome electrode 13 whichis in many respects similar to main dome electrode 12, except that inthe central portion of the hemisphere, an aperture 14 is defined thereinfor purposes that will be described below.

Each of dome electrodes 12 and 13 has a collar forming a base to affordmeans for brazing to the bottom and top edges of the ceramic bodyinsulator 11. Disposed between the collar of main dome 12 and insulator11 is a thin washer of brazing metal used for brazing the collar of theelectrode 12 to the bottom edge of the cylindrical body insulator 11. Insimilar fashion, the collar of the trigger dome is brazed to the topedge of the cylindrical insulator 11 through the medium of a thinmetallic washer of the same metal as that used for securing electrode 12to the ceramic body insulator 11. Except for the aperture 14 located inthe center of trigger dome 13, and a small hole 29 to permit easyloading of gas during the gas filling process, the unitary structureformed by the body insulator 11 and the electrodes 12 and 13 would beair tight. The general configuration resulting is that the two hollowhemispheres 12 and 13 are disposed inside the hollow insulator 11, withthe hollow hemispherical main dome 12 being disposed concave downwardand trigger dome 13 concave upward. The distance in the tube between theclosest points or the surfaces of domes 12 and 13 determines thebreakdown voltage of the tube (in conjunction with the type of gasfilling the tube and its pressure).

Of considerable importance is the shape or contour of the inner surfaceof the ceramic body insulator 11. This shape may best be seen in FIGURE2, which shows a cross-sectional view of the triggered spark gap shownin perspective in FIGURE 1. This cross-sectional view is taken bypassing an imaginary plane diametrally through the triggered spark gap,so as to coincide with, and be parallel to, the longitudinal axis of thecylindrically shaped ceramic body insulator 11. The contour of the innerface of the body insulator 11 is of importance for reasons that will bedescribed more fully below. A description of the shape itself is allthat will be described at this point.

The outside surface of insulator 11 defines a right cylindrical shape.The inner surface of insulator 11 defines a right cylindrical surfaceonly along approximately the middle half of the insulator, i.e., betweenthe imaginary lines 1546. Above line 15 and below line 16, the innersurface radically departs from the straight right cylindrical surface ofits middle portion. This is due to the ringlike extensions 17 and 18 tothe bottom and top edges of the insulator 11, respectively. The bottom(outside) face of ring 17, and the top (outside) face of ring 18 formthe flat surfaces to which the collars of the insulators 12 and 13 arebrazed, respectively. The inside surfaces of ring-like extensions 17 and18 (i.e., the surfaces inside the tube) are shaped in a special way. Asseen in the cross-sectional view of FIGURE 2, the inside surface of ring17 is defined by a curved line which commences at main dome 12 andproceeds along an arc of a circle (having a radius of curvature which isthe same for both 17 and the top ring 18) to meet the middle portion ofthe inner cylindrical surface of insulator 11 substantiallytangentially. The cylindrical surfaces of 17 and 18 are not deliberatelyin intimate contact with electrodes 12 and 13, but due only tomechanicaltolerances. The inner surface of ring 18 is similarly arrangedrelative to trigger dome 13. This internal shaping of the body insulator11 relative to the dome electrodes 12 and 13 is to simu-v late (at theinner surface of insulator 11) the electric field pattern between thedomes 12 and 13 when the domes- 12 and 13 are mounted in free space anda potential is applied therebetween. Thus, immediately adjacent theinner surface of insulator 11, the lines of electrical force commencingand terminating on domes 12 and 13 are of substantially the same shapeas the inner surface of the insulator 11.

Attached to the brazed collars of dome electrodes 12 and 13 respectivelyare electrical terminals 19 and 20 respectively, to which may be appliedthe potential which is to be switched by the spark gap.

Trigger dome electrode 13 and main dome electrode 12 are made of amaterial characterized by a high melting point and low vapor pressure.This is necessary, because when the spark gap is activated, its internaltemperature may reach to approximately 6,000 C. Tantalum is a metalwhich well satisfies these requirements and the additional requirementthat it be easily brazed to the ceramic insulator 11. Molybdenum,tungsten and columbium are examples of other metals which may be usedbecause they have similar properties.

The third electrode of the spark gap is the trigger electrode 21 (asdistinguished from the trigger dome electrode 13). It is disposed in theaperture 14 formed in trigger dome electrode 13. The trigger electrode21 is disposed coaxially with the cylindrical portion of insulator 11.Extending upwardly from the trigger electrode 21 is a trigger tube 22within which the trigger electrode 21 fits. A ceramic cap insulator 23,with a hole through its center for receiving the trigger tube 22, ismounted on top of the spark gap, so that the rim portion of capinsulator 23 may be brazed to the top surface of the collar of triggerdome electrode 13. A thin washer-like element is disposed between therim of cap 23 and the collar of dome 13 to provide the materialnecessary for brazing the two together. The hole through the center ofceramic cap insulator 23 is closed by brazing its boundary to the outerperiphery of trigger tube 22 with a thin wire of brazing material. Byvirtue of the ceramic cap 23 being brazed at these areas, the entirevolume within the spark gap body itself is air tight. This volume isfilled with nitrogen at a pressure of from approximately one-half of anatmosphere to one atmosphere. The precise pressure of the nitrogen (inconjunction with the spacing between the trigger dome 13 and the maindome electrode 12) determines the breakdown potential of the spark gap.Other gases and combinations of gases, such as helium/nitrogen andkrypton 85/ nitrogen may be used.

Trigger electrode 21 is preferably of tungsten, which also has a lowvapor pressure and high melting point. Ceramic cap insulator 23 may beof the same material as the body insulator 11. The trigger tube 22 ispreferably of Kovar or nickel, although materials such as copper orcombinations such as copper/Kovar may be used.

Although trigger electrode 21 is disposed in the aperture 14 of triggerdome electrode 13, it does not fill the aperture. Surrounding triggerelectrode 21 and contiguous thereto, and otherwise filling the aperture14, is an insulator 24 hereinafter referred to as the trigger insulator.Trigger insulator 24 is a ceramic material which may be of the same typeas the body insulator 11 and/or ceramic cap insulator 23.

Of considerable importance is the fact that trigger insulator 24 isgrooved. The annular gap or groove 25 is in the face 26 of the insulatorwhich is perpendicular to the longitudinal axis of trigger electrode 21,and faces main dome electrode 12. The nature of the annular groove 25may perhaps best be seen from the view shown in FIG- URE 3, which is atransverse cross-section taken along a plane perpendicular to thelongitudinal axis of body insulator 11, e.g., along line 33 of FIGURE 2.Groove 25 is circular in form with trigger electrode 21 as its center.Thus, groove 25 forms a circle which is concentric with the boundary ofaperture 14 in trigger dome electrode 13. So that appropriateproportions may be visualized (keeping in mind that the externaldiameter of the body insulator 11 may be approximately .675 of an inch),the width of groove 25 may be approximately .015 of an inch (the widthbeing the dimension in the plane of face 26 of insulator 24). The depthof groove 25 may be approximately .030 to .045 of an inch. Theimportance and function of groove 25 in the operation of the spark gapwill be discussed in some detail below.

The lower tip of trigger electrode 21 is round in shape to form abullet-like head. It is also highly desirable to uniformly roughen thesurface of trigger dome electrode 13 prior to use. This helps decreasesputtering of the cathode electrode metal. It is important that theroughening be done uniformly over the surface of dome 13.

Nitrogen gas may be introduced into the body of the spark gap through ahole in the side of tube 22 and a hole 29 in dome electrode 13.

The operation of the triggered spark gap and the problems involved insuch operation may now be properly comprehended. In operation, a voltageis applied across the main dome electrode 12 (the anode) and the triggerdome electrode 13 (the cathode). This voltage is somewhat less than thatrequired to cause normal or static breakdown. Another voltage is thenapplied between trigger electrode 21 and the trigger dome 13 to initiatebreakdown between the dome electrodes 12 and 13. In the gas dischargethat occurs, electrons are emitted from trigger dome 13, andelectron-ion pairs are formed in the nitrogen gas between theelectrodes. The current density at the cathode 13, during the arcdischarge, is extremely high and causes some vaporization of the cathodemate rial. This, in conjunction with the ionic bombardment of thecathode, results in an erosion of the cathode material and eventualdeposition of the eroded metallic material elsewhere in the interior ofthe spark gap tube. Typically, the metallic deposition, which in theexample given is tantalum, forms on the inside surfaces of ceramic bodyinsulator 11, and also on the face 26 of insulator 24.

Prior to the instant invention, the metallic vaporization and subsequentdeposition was so thick that there was a change in the inter-electroderesistance as a function of length of operation. The deposited metallicfilm also distorted the electric field adjacent to the electrodes andwhile the film was being formed, some gas molecules were undoubtedlytrapped by the Blodgett-Vander slice phenomenon. The latter effectcauses the reduction of gas pressure through clean-up. The longer thetube was operated and the more the cathode electrode was eroded, thegreater the amount of metallic deposit formed, and consequently thegreater were the harmful effects. Needless to say, heavy deposition oftantalum on the inside face of the body insulator between the main andtrigger domes drastically affected the hold-off performance of the tube.Similarly, metallic deposition across the insulator spacing the triggerelectrode from the trigger dome electrode had a considerable effect uponthe resistance therebetween, and therefore affected delay time andjitter.

These past undesirable effects are completely eliminated by the circulargroove 25 in the trigger insulator 24 disposed between the triggerelectrode 21 and trigger dome electrode 13 and minimized by thespecialcontoured shape of the inner surface of the body insulator 11.

Consider first the function of the annular groove 25 in the triggerinsulator 24. It has been ascertained that with the groove 25 disposedbetween trigger 21 and the trigger dome 13, any deposition that theremay be of material on the insulator 24 fails to form a completedelectrical path between the electrodes. Although eroded metal may, infact, form on the insulator 24 between trigger 21 and dome 13, andalthough the deposited metal may penetrate into groove 25 along itssides (see FIG- URE 4) it has been clearly ascertained that thevaporized metal does not, and apparently cannot, penetrate all the wayto the bottom edge 28 of groove 25.

As may be seen in the enlarged view of FIGURE 4, a thin metallic film 27has been formed, due to erosion of the electrode 13. This thin filmcommences at the trigger tube electrode 21 and crosses over face 26 ofinsulator 24 to the region of groove 25. Similarly, there is a thindeposition of metal commencing at trigger dome 13 and passing over theface of insulator 24 to groove 25. It may be noted that the depositionof metal takes place on the two inside parallel surfaces of groove 25,but does not enter into the groove very deeply. In fact, the entrance ofthe deposited metal is no more than about onethird the total depth ofthe groove. The remote wall or base 28 of the groove is entirely free ofany deposited metal. Therefore, electrodes 21 and 13 remain insulatedfrom each other.

Why the vaporized metal does not penetrate all the way into groove 25 isnot clear. One plausible explanation is that the nitrogen gas iscompressed in groove 25 by, and serves to act as a cushion against, theionic bombardment and metallic vapor trying to enter the groove. Thus,the tendency of the ions and metallic vapor to push into the groove 25is resisted by a type of cushioning due to compression of the nitrogenagainst the remote wall 23 of the groove.

Minimization of metallic vaporization from the cathode electrode 13 isaccomplished by the spatial configuration and relation of electrodes 12and 13 and the contoured inner surface of body insulator 11. If auniform electrical field pattern exists between dome electrodes 12 and13, then the discharge phenomenon that takes place therebetween wouldnot tend to be concentrated in one particular area. Thus, if the twoelectrodes were suspended in free space and the geometry of theelectrical field pattern therebetween were not distorted or compressedin any way because of any boundary conditions other than those definedby the conductive surfaces of the electrodes themselves, then therewould be no tendency for the discharge to establish itself at anyparticular place on the electrodes. Once, however, there is an enclosureformed about the electrode, as, for example, by the body insulator 11,problems arise with respect to the uniformity of the electrical fielddistribution between the electrodes.

The body insulator is a dielectric material which, in the example givenfor the insulator 12, may have a dielectric constant of 9. The inside ofthe tube is filled with nitrogen and so the dielectric constant withinthe tube and immediately adjacent the inside face of insulator 11 isapproximately that of free space, or one. Thus, a dielectric interfaceis defined at the inside surface of insulator 11. Such an interfacetends to distort the normal electric field pattern between electrodes 12and 13. Such a distortion could, and it is believed in the past it has,resulted in an undesirable concentration of electric lines of force inrestricted areas of the electrodes. As a consequence, the initiation ofbreakdown has occurred in an unpredictable, undesirable manner. Inaccordance with the principles of the invention, however, the insideface of the body insulator 11 has been contoured and shaped to conformto the geometry of the electrical lines of force that would normallyappear between the electrodes 12 and 13 if they were, in fact, suspendedin free space and not bounded. Furthermore, the walls formed by the bodyinsulator 11 have been spaced as far from the electrodes as possible,consistent wtih other practical structural and mechanical factors. Bymaking the bounding dielectric interface conform to the shape of theelectrical field pattern at the interface, the electrical field Patternis left relatively undistored. This, it is believed, avoids thegeneration of unusually high temperatures in isolated regions on thecathode electrode and thus tends to greatly minimize the amount ofmetallic deposit on the insulators within the tube. The shape of theinner face of the body insulator is such that a line drawn on it fromone electrode to the other electrode describes the path of an electricalline of force at that region between the two electrodes. It is a curvedline because the two electrodes are hemispherical in shape, and as isknown in the art, the electrical lines of force commencing from eitherof the two conductive boundaries must be at right angles to the surfacesthereof.

The above heuristic explanation for the improvement in operation due tocontouring the inside surface of insulator 11 is the best presentlyavailable. However, no definitive theory has been developed whichexplains the success of the invention.

Contouring the inner surface of insulator 11 in this manner provides anon-electrical advantage as well. The rim-like portions 17 and 18 ofinsulator 11 used to form the appropriate contour also provide thickenedportions at the top and bottom of the insulator. This adds mechanicalstrength to the tube envelope and aids in the brazing of the domeelectrodes 12 and 13 to insulator 11.

While particular embodiments of the invention are shown, it will beunderstood that many modifications may be made without departing fromthe spirit thereof, and it is contemplated by the appended claims tocover any such modifications as fall within the true spirit and scope ofthe invention.

What I claim is:

1. A triggered spark gap tube comprising a trigger dome electrode, atrigger electrode, said electrodes being adapted to have a difference ofpotential applied between them during at least part of the operation ofsaid tube, and means disposed between said electrodes for preventing thecreation of a non-gaseous electrical conduction path between saidelectrodes during operation of said tube, said means comprising anelectrical insulator surrounding and contiguous with said electrodes,having an annular groove disposed wholly in the surface of saidinsulator, said annular groove comprising substantially parallel sidewalls over the depth of said groove.

2. A triggered spark gap tube comprising a trigger dome electrode, atrigger electrode, said electrodes being adapted to have a difference ofpotential applied between them during at least part of the operation ofsaid tube, and means disposed between said electrodes for maintainingthe inter-electrode resistance between said electrodes substantiallyconstant over the operation life of said tube, said means comprising anelectrical insulator surrounding and contiguous with said electrodes,having an annular groove disposed wholly in the surface of saidinsulator, said annular groove comprising substantially parallel sidewalls over the depth of said groove.

3. A triggered spark gap tube comprising a trigger dome electrode, arod-like trigger electrode, said electrodes being adapted to have adifference of potential applied between them during at least part of theoperation of said tube, a ceramic insulator surrounding and contiguousto the longitudinal surface of said rod-like trigger electrode and incontact relationship with a portion of said trigger dome electrode, saidceramic insulator having an annular groove formed therein and concentricwith said rod-like trigger electrode, said annular groove having sidewalls substantially arallel over the depth of said groove, said sidewalls being substantially parallel to the longitudinal axis of saidrod-like trigger electrode.

4. A triggered spark gap tube comprising a hollow substantiallyhemispherical metallic electrode with a circular aperture therein, ametallic rod-like electrode extending into the center of said aperture,a ceramic insulator surrounding and contiguous to said rod-likeelectrode and substantially filling the rest of said aperture in saidhemispherical electrode, and an annular groove having substantiallyparallel side walls formed in said ceramic insulator and concentricwithin the boundary of said circular aperture.

5. A triggered spark gap tube recited in claim 4 wherein said groove isin a surface of said ceramic insulator facing the center of said sparkgap tube.

6. A triggered spark gap tube comprising a trigger dome electrode, atrigger electrode, said electrodes being adapted to have a difference ofpotential applied between them during at least part of the operation ofsaid tube, an electrical insulator disposed between and contiguous withsaid electrodes, and means for maintaining the inter- 9 electroderesistance between said electrodes substantially constant over theoperating life of said tube irrespective of metallic deposition uponsaid insulator comprising an open annular groove having substantiallyparallel side walls disposed wholly in the surface of said insulatorbetween said electrodes.

7. A triggered spark gap tube comprising first and second opposing domeelectrodes disposed within an envelope, said dome electrodes beingadapted to have a diiference of potential applied therebetween, saidfirst dome electrode having an aperture therein, a trigger electrodedisposed within said aperture, said trigger electrode and said firstdome electrode being adapted to have a dilference of potential appliedtherebetween, a non-gaseous insulator disposed between and contiguouswith said first dome electrode and said trigger electrode, saidinsulator having an annular groove disposed in its surface andconcentric to the boundary of said aperture, said annular groove havingsubstantially parallel side walls and a depth sufficiently great toprevent the penetration of metallic vapor particles to the remotesurface of said groove during the operation of said tube to thereby pre-10 clude varying the electrical resistance between said first domeelectrode and said trigger electrode during the operation of said tube.

References Cited by the Examiner UNITED STATES PATENTS 1,352,089 9/20Schmidt 313-136 X 1,498,420 6/24 Bennett 313-326 X 1,679,449 8/28 Smith313-216 1,824,452 9/31 Warnser 313-356 X 1,850,585 3/32 Hendry 313-356 X2,514,165 7/50 Ramsey 313-204 X 2,564,040 8/51 Vance 313-217 2,762,9459/56 Berghaus 313-204 X 3,087,092 4/63 Lafierty 313-188 X FOREIGNPATENTS 809,323 7/51 Germany.

GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, ARTHUR GAUSS, Examiners.

1. A TRIGGERED SPARK GAP TUBE COMPRISING A TRIGGER DOME ELECTRODE, ATRIGGER ELECTRODE, SAID ELECTRODES BEING ADAPTED TO HAVE A DIFFERENCE OFPOTENTIAL APPLIED BETWEEN THEM DURING AT LEAST PART OF THE OPERATION OFSAID TUBE, AND MEANS DISPOSED BETWEEN SAID ELECTRODES FOR PREVENTING THECREATION OF A NON-GASEOUS ELECTRICAL CONDUCTION PATH BETWEEN SAIDELECTRODES DURING OPERATION OF SAID